black hole formation?????

[dumbbloke]

Tell me where I go wrong in this, pls.

 

1. gravity is related directly to the magnitude of the relevant masses and inversely to the distances squared, NOT the density of the masses.

 

2.

A large star, of mass m, and distribution of mass radius r1 starts to cool, and eventually collapses into itself to form a super-dense material-still of mass m, but now of a smaller radius of distribution of mass r2. [Laws of Conservation]

 

Q. How can gravity of the collapsed star change to attract other objects into its black hole, unless it didn't because it was a pre-existing property of the gravitational field?

 

3.

The gravity field strength of the collapsed star is now at a net further distance from any of it's solar companions, thus weakening the overall graviational effect and allowing them to slowly drift off.

[zapatos]

Assuming the star lost no mass in its collapse, the gravity field strength as measured by its companions remains the same. From the perspective of a nearby star, the distance to the source of the gravitational field has not changed: it is the center of mass of the now collapsed object, which was also the center of mass of the star.

 

The change in gravitational force is noticed in the distance between what was once the 'surface' of the star, and what is now the 'surface' of the compacted mass. If you remain outside the diameter of the original star, you notice no change in gravity. If you move inside the diameter of the original star, since you are getting closer to the center of mass, you will notice a stronger gravitational field.

Edited May 16, 2013 by zapatos

[dumbbloke]

There is ZERO gravity in the centre of a planet or star! .the gravity of an object is measured from its nearest distribution of mass.

BTW thanks for confirming the grav force doesn't change. So how do these people get a black hole from a star that didn't have the property prior to collapse, as I was taught many moons ago.

[imatfaal]

There is ZERO gravity in the centre of a planet or star! .the gravity of an object is measured from its nearest distribution of mass.

 

BTW thanks for confirming the grav force doesn't change. So how do these people get a black hole from a star that didn't have the property prior to collapse, as I was taught many moons ago.

 

The energy produced from the fusion of hydrogen to helium keeps the material of the star very hot - this manifests in an outward expansionary pressure which resists the compacting gravity. Whilst the star is hot (firstly from hydrogen fusion and later in its life from fusion of heavier elements) the average kinetic energy of the particles that make up the star is so high that the star resists the collapse due to gravity - but as the star uses up the hydrogen it has to use elements that provide less energy and are less likely to fuse. At some point in this process (depends on size and heat of the star) the outward pressure due to the massive heat is no longer enough to stop the star compressing; this often starts up a new set of fusion of heavier elements (the collapse causes heating) - but this is only prolonging the story. The collapsing matter gains momentum inwards, it races towards the centre getting faster as it moves inwards, and the star implodes - this is a nova. The core remnants after the nova are very dense, and comparatively cold - if the mass of the remnant core is over 3-4 times the mass of the sun there is no longer anything to stop its own mass from drawing itself inwards and inwards. It is this collapsing core that can form a black hole. There are other forms of black hole (primordial) but I thought this was the form you were asking about.

[dumbbloke]

Tks, I gathered most of that. What I want to know is does the gravity of the collaped star differ from the previous expanded gravity, and if not, why do I remember being taught that they then attracted other masses/light into themselves because of gravity. Surely those properties must have pre-existed with the original star, even if they could not be observed, because the gravity is unchanged.

[zapatos]

There is ZERO gravity in the centre of a planet or star! .the gravity of an object is measured from its nearest distribution of mass.

From the perspective of the companion, the center of mass of the star is the same as the center of mass of the black hole.

The center of mass is a useful reference point for calculations in mechanics that involve masses distributed in space, such as the linear and angular momentum of planetary bodies and rigid body dynamics. In orbital mechanics, the equations of motion of planets are formulated as point masses located at the centers of mass. The center of mass frame is an inertial frame in which the center of mass of a system is at rest at with respect the origin of the coordinate system.

https://en.wikipedia.org/wiki/Center_of_mass

[dumbbloke]

<From the perspective of the companion, the center of mass of the star is the same as the center of mass of the black hole.>

 

True, but is gravitation of a sphere measured from its center, or its distribution? The distribution is where it is, the center can be very far away.

[zapatos]

What I want to know is does the gravity of the collaped star differ from the previous expanded gravity...

If the mass remains the same, the gravity remains the same.

...and if not, why do I remember being taught that they then attracted other masses/light into themselves because of gravity.

They DO attract other masses/light into themselves because of gravity. They did before the collapse and they do after the collapse.

 

The difference is, after the collapse, you can get closer to the center of mass while remaining outside the diameter of an object. Since the gravitational force increases as you approach the center of mass, it eventually becomes strong enough (in the case of a black hole) that if light or mass gets close enough, it becomes trapped.

 

Think of it like this. If you are 1 million miles from earth the gravitational pull is relatively small. As you get closer, it strength of the field increases, until it maxes out at the surface of the earth. But if the surface of the earth was 1000 miles closer to the center of the earth, then when you reached THAT surface the gravitational field would be even stronger. If the surface of the earth got close enough to its center of mass, it would become a black hole, and if you got that close to the center of mass, you would never be able to escape.

[dumbbloke]

Thank you, now I get it. The objects are closer to the core than the surface of the star pre-collapse, and therefore getting into a denser grav field, because it's now steeper and they can get closer to the center of mass without collision with the periphery of the star.

Edited May 16, 2013 by dumbbloke

[zapatos]

<From the perspective of the companion, the center of mass of the star is the same as the center of mass of the black hole.>

 

True, but is gravitation of a sphere measured from its center, or its distribution? The distribution is where it is, the center can be very far away.

I don't know if this is precisely correct (and someone else may with to jump in), but my understanding is it is measured from its center. If the sun suddenly shrunk to the size of a marble, we would not feel any difference in the gravitational pull from the sun.

 

Thank you, now I get it. The objects are closer to the core than the surface of the star pre-collapse, and therefore getting into a denser grav field, because it's now steeper and they can get closer to the center of mass without collision with the periphery of the star.

Correct.

[dumbbloke]

yes, and objects which travelled inside the original periphery would be subject to more gravitation, because they were closer without obstruction. in the same total gravity well. IE the gravity at Earth would be the same, but provided you were within the solar periphery and more than the distribution of mass from the core you would experience a higher gravitational attraction due to proximity to the source, which was previously unavailable.

Edited May 16, 2013 by dumbbloke

[zapatos]

yes, and objects which travelled inside the original periphery would be subject to more gravitation, because they were closer without obstruction. in the same total gravity well.

Just to make sure we are saying the same thing, I don't think I'd use the phrase 'without obstruction'.

They get closer to the center of mass without ever passing the surface of the object.

As you rightly pointed out earlier, if you travel below the surface, the gravitational field generally lessens, as you now have some of the mass before you, and some behind you, canceling each other out.

[dumbbloke]

Agreed. thanks for sorting that.

[Wilmot McCutchen]

Collapsar axial jets transport matter away from the black hole. Not everything falls in. The axial jets can result in gamma ray bursts perceived on Earth from sources billions of light years away.

 

This 2012 astrophysics paper by Milosavljevic et al. is entitled "Supernovae powered by collapsar accretion in gamma ray burst sources." http://iopscience.iop.org/0004-637X/744/2/103/fulltext/